Crystallization at high pressure to prevent self diffusion of materials



March 31, 1970 w. D. WILSON ETAL 3,503,717

CRYSTALLIZATION AT HIGH PRESSURE TO PREVENT SELF DIFFUSION OF MATERIALS Filed Dec. 22, 1966 FIG.

COOLING SYSTEM HIGH PRESSURE I GENERATOR FIG.

m." mm mm 0 W PRESSURE wmnkdmwmimk United States Patent 3,503,717 CRYSTALLIZATION AT HIGH PRESSURE TO PREVENT SELF DIFFUSION OF MATERIALS Wayne D. Wilson, 2000 Wallace Ave., Silver Spring,

Md. 20902, and Hubert B. Hall, 716 Somerset Place,

Hyattsville, Md. 20783 Filed Dec. 22, 1966, Ser. No. 605,513 Int. Cl. B01 17/30; C01b 9/08; B01d 9/00 U.S. Cl. 23294 7 Claims ABSTRACT OF THE DISCLOSURE A method of high-pressure crystallizing materials which have a tendency to decompose or self diffuse on heating. The process utilizes a temperature gradient in a high pressure vessel to cause a circulation of the high pressure gas system to deposit vapors from the melt 0f the material to llae crystallized on a seed crystal suspended above the me t.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The present invention generally relates to a method of crystallizing a material and particularly to a method for crystallizing a material which decomposes on heating.

Prior art methods of crystallization include the flame fusion method, the flux method, the hydrothermal method, the epitaxial method, and crystallizing from supersaturated solutions. Other methods commonly used in the crystallization field are growing crystals fromtheir melts by zone heating and pulling crystals from their melts by means of a seed crystal. Although these methods are commonly known and accepted in the art, they are subject to certain shortcomings and limitations. For example, in the flame fusion method, which is a rapid growth method giving large crystals, the crystals have known defects which adversely affect their optical properties. Only certain crystals are suitable for growth in the hydrothermal method since this method requires that the mineral, or oxides of the mineral to be crystallized, be soluble in water as a chemical reaction must occur at the seed crystal. It is diflicult in this method to cause the oxides to dissolve in the proper proportions and then precipitate out on the seed crystal in the proportions desired. In most cases, only small crystals can be obtained by this method. In the epitaxial method, crystals can only be made by the depositions of thin films thus limiting the size of crystals attainable. In the crystallization from the supersaturated solution method it is necessary to closely control the conditions of temperature over a long period of time thereby resulting in a slow and involved process. Crystallization from a melt and by crystal pulling are rapid methods of growing optical quality crystals but these techniques are not applicable to materials which decompose when heated.

Recently techniques have been developed for growing crystals from a melt of the material under high pressures of up to 200 atmospheres. In this method a pressed powder of the material to be crystallized is placed in an inert vessel, heated in an inert atmosphere under a high pressure until it melts and then allowed to cool slowly to grow the crystal. While high pressure crystal growth is beneficial in that it allows crystal growth in all directions and not just as thin films and also retards or prevents dissociation of the material to be grown as a crystal, certain drawbacks exist in that due to the environment of "ice growing the crystal in a capsule there is a certain amount of interference of the crystal growth due to the capsule. Further, in many instances a flux is required in the cap sule method and thus there is a slight contamination of the crystal due to the pressure of the flux. These detrimental eifects are obviated by the process of the instant invention by utilizing the vapors of the melt nucleating on a seed crystal.

SYNOPSIS OF THE INVENTION The method of the present invention improves on the high pressure method in that it utilizes the high pressure inert atmosphere system and suspends a seed crystal over the melt of material to be crystallized. The vapors from the melt are then carried to the seed crystal and there nucleates and grows. This method offers the advantages of not requiring a crucible to hold the crystal while it grows and thus eliminates the chance of the crucible interfering or reacting with the crystal during growth. Also the necessity of using a flux which may otherwise contaminate the crystal and/or interfere with the growth process it obviated.

The present invention therefore provides a method for growing large, single crystals having superior density, hardness and optical properties of materials which tend to dissociate when melted at low pressures in a high pressure inert atmosphere.

Briefly, in accordance with this invention these and other objects are accomplished by melting a material, such as garnet, in a high pressure, high temperature inert atmosphere. A seed crystal is suspended above the melt of the material and the temperature gradient between the melt of material and the seed crystal causes a circulation of the high pressure gas system thus sweeping the vapors from the melted material up to the seed crystal thereby allowing the crystal to nucleate and grow.

1 A more complete appreciation of this invention and many of its attendant advantages will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawing wherein:

FIG. 1 is a sectional view of the apparatus used in the process of crystallization; and,

FIG. 2 is a graph of the melting curve and stability curve for a hypothetical chemical compound.

The process of the present invention is carried out in a high pressure inert gas system consisting of a pressure vessel 10 having a central chamber with a high temperature furnace, shown generally at 12 disposed therein. The sample material 14 to be crystallized is placed within an inert capsule or crucible 16. The crucible 16 is placed within the furnace 12 which consists of a high temperature resistant ceramic tube 18 having a platinum or tungsten wire solenoid 20 encircling it and connected to a suitable energy source (not shown) to enable the coil of wire to heat the furnace and consequently the crucible 16.

The pressure vessel 10 includes an axially deposited high temperature resistant ceramic elongate tube 22 lining the inner wall 24 of the pressure vessel 10 to shield the pressure vessel from the high temperatures generated by the furnace 12 placed therein. To cool the pressure vessel 10 from the effects of the furnace, a cooling system, shown generally at 26, is provided which includes a series of concentric tubes 28 disposed Within the wall 24 of the pressure vessel adjacent the ceramic tube 22 to allow cooling fluid, such as cold water, alcohol or some other refrigerant, to circulate, In the alternative, the pressure vessel may be cooled by placing a grooved inner liner in the vessel and allowing the coolant to circulate through the grooves which exist between the vessel and the liner.

Cooling may also be affected by circulating the high pressure gas used to pressurize the vessel.

The apparatus is also provided with a conventional high pressure generator system 30 and including a suitable connection 32 to the interior of the pressure vessel to allow an inert gas such as argon, nitrogen or the like to pressurize the interior chamber of the pressure vessel 10 and consequently the furnace and the sample material to be crystallized. Suitable high pressure retaining plugs 34 are provided to effectively seal the pressure vessel 10.

The Wall 36 of ceramic tube 18 extends upward above the crucible 1-6 to the underside 38 of the top plug 34 so that a cylindrical space 40 is created between the top of crucible 16 and the underside of plug 34. The seed crystal 42 for the material to be crystallized is suspended in any convenient manner in the space 40.

The general method of operation is to place a sample of the material to be crystallized within the crucible 16. The sample may be in powdered form or it may be the oxides of the material to be crystallized or material that has been presintered to form a glass or polycriptalline mass. The crucible is then positioned within the furnace 12 and the seed crystal is suspended above the crucible within the enclosed space 40. The vessel 10 is then pressurized within a range of from 7.5 to 15 kilobars, depending on the properties of the material to be crystallized, to prevent the material from dissociating when heated. The temperature is then raised by means of the furnace 12 to melt the sample in the range of 1400 C. to 2000 C. depending on the melting temperature of the material. The temperature rise will naturally cause a pressure rise, therefore pressure and temperature must be selected accordingly. For example, if an initial pressure of kilobars is used and then the temperature is raised to 1000 C., the resulting pressure may be as high as kilobars.

It is to he expected that there will be no vapor pressure above the melted sample of material when the applied pressure within the vessel exceeds the vapor pressure of the material at the operating temperature. If this were the only operative phenomenon, crystals could not be grown from the vapor at high pressures. However, in the present method, there is a strong temperature gradient between the upper end of the crucible and furnace and the underside 38 of the vessel plug 34. This gradient amounts to as much as 400 C. per inch. This temperature gradient within the space 40 will cause a circulation of the high pressure gas system (shown by the circular arrows) at gas speeds of as much as 100 miles per hour between the cool end of the plug 34 of the pressure vessel and the hot end of the crucible. This circulation of the swiftly moving gas over the melted contents of the crucible will sweep vapors from the liquid upward toward the cooler end plug of the vessel and over the seed crystal 42 mounted in the space 40 so as to condense and grow onto the seed in the desired manner. The rate of growth depends upon the position of the crystal and upon the temperature gradient.

After a suitable length of time has elapsed (on the order of a few hours) the temperature is decreased and the pressure released to atmospheric conditions. It is essential that the pressure be held above certain critical conditions as the temperature is reduced to. prevent the crystal from decomposing to some other form. Since the pressure will decrease as the temperature is reduced it is necessary to raise the pressure within the vessel so as to maintain a constant pressure level until cooling is completed.

The situation for materials which may be grown by this technique is shown by the graph in FIG. 2. It is noted that temperature and pressure are plotted as the axes and the melting curve and stability curve for the material to be crystallized are shown. For any pressure shown the material melts above the melting curve and decomposes above the stability curve. Consequently, in this system, melting must be carried out above the melting curve and below the stability curve in the region A shown in FIG. 2. Crystallization of the vapors will then occur at temperatures below the melting curve and also below the stability curve.

Among those materials that can be crystallized by the instant method are the fluorides, molybdenates and tungstenates of lead, nickel and the Group I and II metals of the Periodic Table of the Elements, and also the garnets. Those particularly suited are LiF, CaF BaF MgF CdF SIFZ, CsF, NaF, PbMoO CaMoO NiMoO and CaWO Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

We claim: 1. A method of crystallization which comprises pressurizing a quantity of material sufficiently to prevent self diflusion of the constituents of the material to be crystallized,

heating said material to a temperature above its melting points while maintaining the applied pressure,

maintaining a decreasing temperature gradient above the melted material sufficient to cause turbulent gas flow above the melted material,

suspending a seed crystal above the melt of the material to be crystallized in the zone of turbulent gas flow, and

allowing said turbulent gas flow to pass the vapors from the melt of material over said seed crystal to thereby cause the crystal to nucleate and grow.

2. The process of claim 1 wherein said pressurization is carried out by means of fluid pressure.

3. The process of claim 1 wherein said pressurization is carried out in the range of 7.5 to 15 kilobars and said temperature is in the range of 1400 C. to 2000 C.

4. The process of claim 1 wherein said temperature gradient above the melted material is on the order of 400 C. per inch.

5. The process of claim 1 wherein the material to be crystallized is selected from the group consisting of the garnets, the fluorides, molybdenates and tungstenates of lead, nickel and the Group I and II metals.

6. The process of claim 1 wherein the material to be crystallized is selected from the group consisting of LiF, CHFZ, B8132, MgFz, CdFg, S F 'CSF, NAF, PbMOO4, CaMoO NiMoO CaWO and the garnets.

7. The process of claim 1 including the step of cooling the crystal while maintaining a high pressure to prevent decomposition of the crystal.

References Cited UNITED STATES PATENTS 2,808,312 10/1957 Rudge 23294 XR 2,842,468 7/ 1955 Brenner 23294 XR 2,929,691 3/1960 Decroly 23-294 XR 3,230,051 1/1966 Sampson 23301 XR 3,273,969 9/1966 Sirgo 23294 XR 3,030,187 4/1962 Eversole 23209.l 3,124,422 3/1964 Custers et al. 23209.l 3,175,885 3/1965 Brinkman et al 23209.1

. FOREIGN PATENTS 679,054 1/ 1964 Canada.

NORMAN YUDKOFF, Primary Examiner C. RIBANDO, Assistant Examiner US. Cl. X.R. 23209.1, 301 

